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Carbohydrates are one of the most structurally and functionally diverse classes of naturally occurring compounds and it is well established that they play an essential role in a vast array of biological processes. The synthesis of stereochemically defined oligosaccharides by a series of glycosylation processes, involving the reaction between a glycosyl donor and acceptor, is of paramount importance in synthetic carbohydrate chemistry and glycobiology. However, despite the importance of glycosylation chemistry and the development of sophisticated methodologies, there remains no general and stereoselective strategy that has been universally adopted for the syntheses of oligosaccharides. In this thesis we present the synthesis and function of a novel family of glycosyl donors, in which fluorinated pyridine systems are utilised as the leaving group. In systems of this type it proved possible to 'tune' the glycosylation capability of the donor via variation of the substituents present on the pyridine ring and the type of Lewis acid activator used. The formation of a glycosidic bond with control of the stereochemistry at the anomeric centre is usually difficult. Interestingly glycosyl donor systems of this type provide a high degree of stereoselectivity, providing diastereomeric excesses in the region of 80 to 98%. It has been determined that polyfluoro-pyridyl glycosyl donors do not react via the established Sn1 glycosylation process but via an unique Sn2 process which gives rise to the high degree of stereoselectivity observed.

Cellulose triacetate was prepared from cotton cellulose employing a heterogeneous method for acetylation, and using zinc chloride as a catalyst. Viscosity-concentration studies for solutions of cellulose triacetate in the mixed solvents, chloroform-acetic anhydride, methylene chloride-acetic anhydride and tetrachloroethane-acetic anhydride were carried out. Molecular weight determinations were carried out on the prepared samples of cellulose triacetate using a high speed membrane osmometer (Hewlett Packard Mechrolab Model 502). The Mark-Houwink viscosity-molecular weight relationship was investigated for the above systems and the respective K and a values were evaluated. The degradation of cellulose triacetate in the solvents chloroform, methylene chloride and tetrachloroethane in the presence of acetic anhydride was investigated and the separate effects of nature of catalyst, catalyst concentration, nature of solvent and temperature on the rate of degradation were studied. The extent of degradation was followed by the decrease in viscosity with time, and molecular weights were evaluated using a singlepoint viscosity relationship to determine limiting viscosity numbers. The catalysts used were ferric chloride, antimony pentachloride, stannic chloride, tellurium tetrachloride, sulphuric acid and perchloric acid. The degradation was shown to be a first order random process which was explained by a mechanism based on the acetylium ion, (CH[3]CO[+]), being the activating catalytic species. Scission of 1-4 oxygen linkages between repeating units is believed to take place with the simultaneous introduction of acetate groups due to reaction of acetic anhydride with the activated complex. Differences in rate constants and activation energies obtained are discussed on the basis of the proposed mechanism, as well as possible differences in chain configuration of cellulose triacetate in the various solvent mixtures.

Mannan A: 1. Mannan A was extracted from delignified ivory nut shavings with 7;., potassium hydroxide solution, and the polysaccharide precipitated by acidification of the extract and addition of methylated spirits. 2. The mannan, purified by means of the copper complex, gave on hydrolysis 97.6% mannose, 1.8% galactose, and o.8% glucose. 3. The reducing power of the mannan determined by oxidation with alkaline hypoiodite gave a degree of polymerisation of 100, and by treatment with 3 :5- dinitrosalicylic acid, 28. The values of the chain length of mannan A obtained by these two methods were different and disagreed with the value obtained by other methods. Values of the chain length of the polysaccharide which are derived from measurement of the reducing power are, therefore, of doubtful validity. 4. A triacetate was prepared which was found by Barger's method to have a degree of polymerisation of 10 -13. 5. The mannan was methylated and the methylated polysaccharide hydrolysed. The hydrolysate was separated on a cellulose column and the following sugars were obtained: 2:3:4:6- tetramethyl Dmannose, 2:3:4 :6- tetramethyl D- galactose, 2:3:6 -trimethyl Dmannose, 2 :3 :4- trimethyl D- mannose, and 2:3- dimethyl D- mannose in the molar ratios of, respectively, 1.0: 0.2: 11.6: 1.0: 0.2. All the galactose present in the purified mannan was accounted for as 2:3:4 :6- tetramethyl galactose. 6. The 2:3 :4- trimethyl mannose fraction crystallised as a disaccharide, which was shown to be 1 -[ 2:3:4 -trimethyl D-mannopyranosido] - 2:3:4 -trimethyl D-mannopyranoside. 7. The degree of polymerisation of the methylated mannan was determined by Barger's method and found to be 9 -11. 8. Periodate oxidation experiments on the mannan showed that 1.6 moles of periodate were consumed per anhydrohexose unit, and one mole of formic acid was liberated per 4 anhydrohexose units. 9. On the basis of these results possible structures for the mannan are proposed. Mannan B: 1. Mannan B was extracted from the residue left after removal of mannan A from ivory nuts. The residue was extracted with cuprammonium hydroxide and the solution treated with sodium hydroxide. Several fractions were obtained and one which was considered to be mannan B was purified by dissolution in anhydrous formic acid and precipitation with ethanol. 2. Purified mannan B gave on hydrolysis 98.3% mannose, 1.1% galactose, and 0.8% glucose. 3 The reducing power of the mannan was determined by hypoiodite oxidation and colorimetrically by means of 3:5- dinitrosalicylic acid. The first method gave an apparent degree of polymerisation of 12, the second, 130. The values of the Chain length of mannan B obtained by these two methods were different and disagreed with the value obtained by other methods. Values of the chain length of the polysaccharide which are derived from measurement of the reducing power are, therefore, of doubtful validity. 4. Mannan B was meth lated and the methylated polysaccharide hydrolysed. Separation of the mixture of sugars on a cellulose column gave 2:3:4:6 -tetramethyl D- mannose, 2:3 :4 :6- tetramethyl D- galactose, 2:3:6 -trimethyl D- mannose, 2:3:4 -trimethyl D- mannose, and 2:3- dimethyl D-mannose in the molar ratios of, respectively, 1 :1:63 :11 :1. All the galactose present in purified mannan B was accounted for as 2:3:4:6 -tetramethyl galactose. 5. The 2 :3 :4- trimethyl mannose fraction crystallised as a disaccharide, which was shown to be 1 -[2 :3 :4- trimethyl D- mannopyranosido ] - 2:3:4- trimethyl D- mannopyranoside. 6. The degree of polymerisation of methylated mannan B was determined by Barger's method and found to be 35-43. 7. Periodate oxidation experiments on the mannan showed that 0.98 moles of periodate were consumed per anhydrohexose unit, and one mole of formic acid was liberated per 2.4 anhydrohexose units. 8. On the basis of these results possible structures for the mannan are proposed.

Algae differ from organisms such as bacteria, fungi and protozoa in having a chemical economy based upon photosynthesis, in which the accumulation rather than the breakdown of organic matter predominates. In bulk of material involved this algal type of metabolism perhaps exceeds any other. The total yield of carbohydrate synthesised in the oceans, in which algae are the only photosynthetic organisms, has been estimated to be from 1.6 to 15.5 x lO10 tons of carbon fixed per year and is evidently at least as much as that from land plants. All animals in the sea must derive their food from this plant synthesis, these algae thus support the fisheries upon which man relies for food. In soil and in freshwaters algal metabolism is generally on a lesser scale, but is nevertheless of considerable importance. Algae contribute to the fertility of soil by nitrogen fixation and as a source of trace elements. The metabolic processes which characterise the algae may be similar to land plants in the initial act of photosynthesis and in many cases similar enzyme systems may be present, however from a study of the structure of the main components there are also metabolic activities peculiar to the algae. Carbohydrate extracts of algae have many present day uses in the drugs, cosmetics, pharmaceuticals, textile, food and fishing industries. Seaweeds have been used as a source of food since ancient times and nowadays are valuable as a source of minerals, trace elements and vitamins for animal feeding stuffs. The possibility of the growth of unicellular algae in culture as a source of human food on prolonged space flights in the solar system is being investigated. The algae may be classified on morphological grounds. The photosynthetic pigments often afford the simplest and most certain means of recognising representatives of different classes.

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